Restaurants and bars use gas, such as carbon dioxide, to carbonate fountain soft drinks and to preserve and push draught beer. Many restaurants have abandoned high-pressure compressed gas cylinders and are now using liquid bulk carbon dioxide as a safer, low-pressure alternative. Liquid bulk carbon dioxide is stored on the premises at a lower pressure in a holding container, and is often refilled on a regular schedule based on a restaurant or bar's usage pattern. The containers are typically a tank-in-tank design, having a rigid outer tank and a rigid inner tank with some amount of insulating space between the outer and inner tanks, and are permanently installed at their respective locations. Bulk carbon dioxide container systems are available in different sizes, ranging from 200 pounds to almost 800 pounds of carbon dioxide capacity, to fit the needs of the individual restaurant or bar.
Carbon dioxide is a compound formed by the combination of carbon and oxygen atoms in a 1:2 ratio expressed by the chemical symbol CO2. The weight percentages of carbon and oxygen are 27.3% and 72.7% respectively. Carbon dioxide is a gas at normal atmospheric temperature and pressure. It is colorless, essentially odorless, and about one and a half times denser than air. Depending on the temperature and pressure to which it is subjected, carbon dioxide may exist in the form of a solid, a liquid, or a gas. At a temperature of −69.90 degrees Fahrenheit and a pressure of 60.43 psig carbon dioxide can exist simultaneously in all three phases. This condition is known as the triple point. At temperatures above 87.90 degrees Fahrenheit carbon dioxide can exist only as a gas, regardless of the pressure. This is known as its critical temperature. Liquid carbon dioxide can only exist in a sealed container between the triple point and critical point temperatures under pressure. There is a definite pressure-temperature relationship of the liquid and gas in equilibrium. Normal operational pressures should remain above 165 psig to prevent the liquid carbon dioxide temperature from dropping below the minimum vessel design temperature. Liquid carbon dioxide should never be stored at pressures below 60.5 psig to prevent the formation of solid carbon dioxide or dry ice.
Carbon dioxide storage tanks are designed for long-term storage of liquefied carbon dioxide. A typical carbon dioxide storage tank is comprised of a steel inner tank encased in an outer steel vacuum shell. The insulation system between the inner and outer containers consists of multiple layer composite insulation and high vacuum to ensure long holding time. The insulation system, designed for long-term vacuum retention, is permanently sealed to ensure vacuum integrity.
A problem often experienced by bulk-fill providers relates to the scheduling of bulk container filling. Holidays or weekends can affect carbon dioxide consumption rates in an irregular manner, making it difficult to accurately predict an out-of-gas situation. This problem is compounded by a common issue where the pre-existing container fill level gauges are broken or inaccurate, although the tanks themselves are otherwise fully functional. A broken fill level gauge can occur when a given container reaches an empty, or nearly empty, state and the container's mechanical internal float is damaged, for example, from the remaining liquid freezing, rendering the fill level gauge inoperable.
Restaurants and bars need to ensure they are able to continue serving beverages to their customers. Bulk-fill providers need to be able to accurately identify containers that need to be refilled, avoiding unnecessary and costly premature fill runs. Therefore, restaurants, bars, and bulk-fill providers alike have a need to accurately, and in some cases remotely, determine the fill-level of their carbon dioxide containers.
Additionally, the need exists for a non-invasive means of measuring fill levels that can be retrofitted to existing containers and bulk-fill systems. Although invasive measuring devices, located within the volume of a container, are well known, the placement of an invasive measuring device within the container's inner tank is often not feasible due to any number of negative factors, including the cost of drilling into the container, the risk of possibly contaminating the liquid or gas disposed therein, the introduction of a source for a possible leak path of the liquid or gas from within the container, or structural issues that could be created by breaching the inner and outer tank's structural walls.
The need also exists for a non-invasive system that can accurately measure the fill-level of containers utilizing a tank-in-tank design. Previous non-invasive means of measuring container fill level having a single tank wall and a flexible interior bladder and have utilized impactors, solenoids, or vibration generators to vibrate the wall surface of the container, detectors to record the directly resulting response vibrations of the wall surface, and a frequency conversion means to convert the recorded data signal to frequency information and determine the peak resonant frequency response. The fill level of the single-wall container is then determined by comparing the measured peak frequency information to stored frequency and volume information for the container. Although this prior art method may have worked for single-wall containers, a measurement of the direct response of a tank-in-tank container's outer tank to vibration does not provide accurate fill level information regarding the inner tank or tank-in-tank container as a whole. Additionally, direct frequency readings of the prior art are affected by mid-range and high-range frequency ambient noise, including the common occurrence of container venting.
It is, therefore, desired that a retrofittable device and method for using the same be provided that is capable of obtaining an accurate measurement of liquid or gas volume within a tank-in-tank container in a non-invasive manner that is not affected by mid-range or high-range frequency ambient noise.